To obtain a one-way lock mechanism applicable to a vehicular seat having an active headrest by a simple constitution.

This cylinder 1 has a cylinder case 1 capable of sealing a working fluid and a piston 2 capable of advancing to and retreating from one end side of its cylinder case 3, and is provided by sealing the working fluid 4 applying resistance to the advance and the retreat of the piston 2 in the cylinder case 3, and has a magnetic viscous fluid as the working fluid and a magnet 10 capable of supplying a magnetic field to the magnetic viscous fluid. The cylinder is characterized by being constituted so that when the piston 2 enters, the supply of the magnetic field to the magnetic viscous fluid 4 from the magnet 10 is stopped, and the viscosity of the magnetic viscous fluid 4 is reduced, and when the piston 2 retreats, the magnetic field is supplied to the magnetic viscous fluid 4 from the magnet, and the viscosity of the magnetic viscous fluid 4 is increased.